Time interlock for air conditioning compressor and the like

ABSTRACT

A circuit for controlling a load, such as the cooling compressor of an air conditioner, to prevent short cycling. A circuit which provides a time delay or interlock after de-energizing a load, before the load can again be energized. A circuit operable to connect a load and a storage capacitor to a power source in response to external switch closure, and on opening of the switch, to disconnect the load and utilize the charged capacitor to block for a period of time, further load switching regardless of external switch operation.

i United States Patent 11 1 Chang TIME INTERLOCK FOR AIR CONDITIONINGCOMPRESSOR AND THE LIKE [75] Inventor: Tom C. Chang, Hollywood, Calif.

[73] Assignee: Cam-Stat Incorporated, Los

Angeles, Calif.

22 Filed: Oct. 18, 1972 21 Appl. No.: 298,463

[52] [1.5. CI. 317/141 S, 317/13 A, 317/36 TD [51 1 Int. Cl. H02h 7/085Field of Search 317/13 A, 13 B, 13 C, 317/40 R, 132-, 141 S, 36 TD;318/432, 445,

[56] References Cited UNITED STATES PATENTS 3,716,756 2 1973 Runge et a1317/13 A TH/\MOSTHT Nov. 20, 1973 5/1972 Pinckaers 317/13 B 6/1971Traina 317/141 S Primary Examiner-J. D. Miller Assistant Examiner-HarryE. Moose, Jr. AttorneyFord W. Harris, Jr. et al.

[5 7] ABSTRACT A circuit for controlling a load, such as the coolingcompressor of an air conditioner, to prevent short cycling. A circuitwhich provides a time delay or interlock after de-energizing a load,before the load can again be energized. A circuit operable to connect aload and a storage capacitor to a power source in response to externalswitch closure, and on opening of the switch, to disconnect the load andutilize the charged capacitor to block for a period of time, furtherload switching regardless of external switch operation.

8 Claims, 1 Drawing Figure SWITCH FL COOL/N6 comm/$530k RELAY (1.0/90)TIME INTERLOCK FOR AIR CONDITIONING COMPRESSOR AND THE LIKE Thisinvention relates to time delay circuits and in particular, to a new andimproved time interlock or time delay circuit which provides apredetermined time interval after de-energization of a load before theload can again be energized. The time delay circuit is especiallyadapted for use in preventing short cycling of air conditioningcompressors but is readily adapted for use in various applicationsrequiring interlock protection.

After an air conditioning compressor has been turned off, it isdesirable that the control system prevent turning on of the compressorfor a period of time, typically three to five minutes, in order toprevent damage and excessive wear. Some cooling systems have a tendencyfor a variety of reasons to turn on and turn off at relatively shortintervals and this undesirable phenomenon is usually referred to asshort cycling. Time delays or interlocks are incorporated in coolingsystems to prevent the short cycling mode of operation.

A variety of devices have been utilized in the past for the time delaycontrol. One form of prior art device utilizes a conventional motordriven timer to provide the desired delay. Another type of prior artdevice utilizes the charging of a capacitor through a resistor, with themagnitude of the delay being a function of the time constant of the RCcircuit. One such prior art circuit is described in Appliance Engineer,Volume 5, No. 5, 1971, p. 24.

While, the resistance-capacitance system does away with the need for themotor driven timer, there are some disadvantages in the present systems.The systems are relatively complex and require the electric power sourceto be on at all times. For example, the five minute delay starts whenthe thermostat switch opens and the compressor is turned off. However,since the time delay results from the charging of the capacitor of theRC circuit, the main power supply must remain on, or the time delay willbe present at a subsequent time when the main power is turned on.Further, the prior art solid state devices are relatively complex andare not readily installed by simple interconnection between thethermostat switch and compressor relay.

Accordingly, it is an object of the present invention to provide a newand improved delay circuit that is simple, small, reliable andinexpensive. A further object is to provide such a circuit which can bedirectly installed between a thermostat switch and a compressor relay.An additional object is to provide a circuit which can be utilized for avariety of delays or interlock applications. I

[t is a particular object of the invention to provide a new and improvedtime delay circuit which does not require any standby power supply toobtain the delay and in particular, one which utilizes dischargingrather than charging to obtain the delay. A further object is to providesuch a system which can be manufactured for any particular desired delayand which will not require timing adjustment.

Other objects, advantages, features and results will more fully appearin the course of the following description. The single FIGURE of thedrawing is an electrical schematic of the preferred embodiment of thepresent invention and is given by way of illustration or example.

In the FIGURE of the drawing, a time delay circuit is connected betweena control switch such as a thermostat switch 11 and a load such as acooling compressor relay 12. The system typically may be energized froma volt A.C. supply with a stepdown transformer 13 providing a lowvoltage such as 24 volts at the secondary winding. One terminal of thesecondary winding of the transformer 13 is connected to the relay l2 anddelay circuit 10 by the switch 11, and the other terminal of thesecondary winding is connected to a common line 15 of the delay circuit.

The delay circuit preferably includes a rectifier and filter combination16 which provides D.C. power at point 17. A switching unit, typically arelay with coil 20 and single pole double throw contact set 21 is connected with the coil 20 in series with a transistor 22 across the D.C.supply 17, 15. The moving arm of the contact set 21 is connected to thecommon line 15, with contact 23 connected to point 24 and with contact25 connected to the relay 12. With the relay coil 20 unenergized, themoving arm of contact set 21 engages contact 23.

A field effect transistor 30 is connected between the base of transistor22 and line 15, with a capacitor 31 connected between point 24 and thebase of transistor 30. A resistor 35 is connected between points 17 and24. A resistor 36 and a diode rectifier 37 are connected between thebase of the transistor 30 and line 15.

The time delay circuit operates in the following manner to provide thedesired interlock function. Initially control switch 11 is open and thedelay circuit 10 and load 12 are disconnected from the voltage source.All capacitors are discharged and all relay coils are unenergized.

When switch 11 is closed, point 24 is connected to line 15 and relaycoil 20 is unenergized. The D.C. potential builds up at point 17 as thecapacitor of the filter 16 charges. The input or base of the fieldeffect transistor 30 is maintained at about 0.6 volts above the commonor circuit ground potential on line 15 due to the clamping action of thediode 37. The transistor 30 is biased at minimum drain source resistanceregion and transistor 22 is immediately driven into saturation. The coil20 is energized and the relay moves the arm of the contact set 21 tofixed contact 25. This switching operation connects the power sourcefrom the transformer 13 through the control switch 11 to the load 12. Inthe illustrated embodiment, the control switch 11 is a thermostat switchand the load 12 is the cooling compressor relay, with thermostat switchclosure producing substantially immediate compressor start.

The switching action produced by energizing the relay coil 20,disconnects point 24 from the common line 15. Capacitor 31 then startscharging through resistor 35 to the potential at point 17, and remainscharged as long as switch 11 remains closed. During this time,transistors 30 and 22 are on and the relay coil 20 and the load 12 areenergized.

When switch 11 is opened, relay coil 20 is deenergized and the movingarm of the contact set 21 switches to contact 23, disconnecting the loadrelay 12 from the power source. Point 24 is connected to the common line15. When switch 11 was closed, tje plus side of capacitor 31 was at thepotential of point 17, say plus 6 volts (with the common line 15 at 0volts). When point 24 is connected to the line 15, the plus side ofcapacitor 31 is dropped to 0 volts with the result that the minus sideof capacitor 31 changes to minus 6 volts, since the capacitor does notdischarge instantaneously.

This negative voltage at the negative side of capacitor 31 and hence atthe gate of transistor 30 biases the transistor 30 into its off regionand turns off transistor 22. Both transistors will remain in the offstate as long as there is a negative voltage bias at the gate oftransistor 30 sufficient to hold the transistor 30 off. Capacitor 31discharges through resistor 36, with the gate voltage of transistor 30rising as the capacitor discharges. The transistor 30 desirably has anextremely high input resistance and the diode 37 desirably has lowleakage reverse current so that the capacitor discharge path isprimarily through the resistor 36, with the time constant of this R.C.circuit and the potential of point 17 determining the duration of delay.The time delay is the time required for the gate potential of transistor30 to rise to a point at which the transistor can be turned on. Ifswitch 11 is closed prior to this time, nothing occurs in the delaycircuit, that is, transistors 30 and 22 remain off and the relay coilremains unenergized. If the switch 11 is closed after the gate potentialof transistor 30 has raised to the turn-on value, the transistors willturn on and the coil 20 will be energized, as previously described.

During the discharge of the capacitor 31, closing and opening the switch11 has no effect on the operation. No standby or other power source isrequired in order to obtain the delay. Closing and opening the controlswitch does not change the duration of the delay or restart the delaycycle. That is to say, the predetermined delay as established by theparameters of the system is achieved and is not shortened or lengthenedby any external factors.

I claim:

1. In a circuit for energizing a load from a power source by a controlswitch not less than a predetermined time after de-energization of theload, the combination of:

switching means operable between first and second conditions;

a control unit for said switching means;

a storage circuit;

a discharge circuit for said storage circuit;

with said switching means when in said second condition connecting saidstorage circuit to the power source through the control switch forcharging said storage circuit from the power source when the controlswitch is closed, and connecting the load to the power source throughthe control switch for energizing the load; and

circuit means connecting said storage circuit to said control unit formaintaining said switching means in said first condition when saidstorage circuit is charged to a predetermined value and said switchingmeans has switched to said first condition; with said switching meansswitching to said first condition when the control switch is openeddisconnecting the power source and with said storage circuit dischargingthrough said discharge circuit and blocking actuation of said switchingmeans to said second condition until discharged to a predeterminedvalue.

2. A circuit as defined in claim 1 wherein said switching means when insaid first condition includes a shunting connection across said storagecircuit.

3. A circuit as defined in claim 1 wherein said switching means includesa transistor connected across the power source and normally biased intoconduction to said second condition by said control unit when thecontrol switch is closed, with said storage circuit blocking saidconduction for a time after the control switch is opened.

4. A circuit as defined in claim 1 wherein said storage circuit includesa capacitor connected across the power source for charging through saiddischarge circuit when said switching means is in said second conditionand connected across said discharge circuit when said switching meansswitches to said first condition, providing a voltage change at saidcontrol unit.

5. A circuit as defined in claim 4 wherein said control unit includes atransistor with the bias thereof provided through said capacitor, withsaid transistor when biased to nonconduction preventing said switchingmeans from switching to said second condition.

6. In a circuit for energizing a load from a power source by a controlswitch not less than a predetermined time after de-energization of theload, the combination of:

a first transistor;

a storage capacitor connected between a first point and the gate of saidfirst transistor;

a resistor and a rectifier connected in parallel between a second pointand the gate of said first transistor;

circuit means connecting said first and second points across the powersource through the control switch; and

switching means controlled by said first transistor for switchingbetween first and second conditions and including means connecting saidfirst point to said second point when in said first condition andconnecting the load to said second point when in said second condition,

with said capacitor charging when the control switch is closed and saidswitching means is in said second condition and discharging when thecontrol switch is opened and said switching means is changed to saidfirst condition.

7. A circuit as defined in claim 6 wherein said switching means includesrelay means with single pole double throw contact means and actuatingelement, and a second current control transistor in series with saidelement across the power source, with said first transistor controllingsaid second transistor.

8. A circuit as defined in claim 7 wherein the power source is an A.C.source, and including a rectifier-filter circuit for connection betweenthe power source and said first and second points providing DC. powerfor said capacitor and switching means.

l =l i

1. In a circuit for energizing a load from a power source by a controlswitch not less than a predetermined time after deenergization of theload, the combination of: switching means operable between first andsecond conditions; a control unit for said switching means; a storagecircuit; a discharge circuit for said storage circuit; with saidswitching means when in said second condition connecting said storagecircuit to the power source through the control switch for charging saidstorage circuit from the power source when the control switch is closed,and connecting the load to the power source through the control switchfor energizing the load; and circuit means connecting said storagecircuit to said control unit for maintaining said switching means insaid first condition when said storage circuit is charged to apredetermined value and said switching means has switched to said firstcondition; with said switching means switching to said first conditionwhen the control switch is opened disconnecting the power source andwith said storage circuit discharging through said discharge circuit andblocking actuation of said switching means to said second conditionuntil discharged to a predetermined value.
 2. A circuit as defined inclaim 1 wherein said switching means when in said first conditionincludes a shunting connection across said storage circuit.
 3. A circuitas defined in claim 1 wherein said switching means includes a transistorconnected across the power source and normally biased into conduction tosaid second condition by said control unit when the control switch isclosed, with said storage circuit blocking said conduction for a timeafter the control switch is opened.
 4. A circuit as defined in claim 1wherein said storage circuit includes a capacitor connected across thepower source for charging through said discharge circuit when saidswitching means is in said second condition and connected across saiddischarge circuit when said switching means switches to said firstcondition, providing a voltage change at said control unit.
 5. A circuitas defined in claim 4 wherein said control unit includes a transistOrwith the bias thereof provided through said capacitor, with saidtransistor when biased to nonconduction preventing said switching meansfrom switching to said second condition.
 6. In a circuit for energizinga load from a power source by a control switch not less than apredetermined time after de-energization of the load, the combinationof: a first transistor; a storage capacitor connected between a firstpoint and the gate of said first transistor; a resistor and a rectifierconnected in parallel between a second point and the gate of said firsttransistor; circuit means connecting said first and second points acrossthe power source through the control switch; and switching meanscontrolled by said first transistor for switching between first andsecond conditions and including means connecting said first point tosaid second point when in said first condition and connecting the loadto said second point when in said second condition, with said capacitorcharging when the control switch is closed and said switching means isin said second condition and discharging when the control switch isopened and said switching means is changed to said first condition.
 7. Acircuit as defined in claim 6 wherein said switching means includesrelay means with single pole double throw contact means and actuatingelement, and a second current control transistor in series with saidelement across the power source, with said first transistor controllingsaid second transistor.
 8. A circuit as defined in claim 7 wherein thepower source is an A.C. source, and including a rectifier-filter circuitfor connection between the power source and said first and second pointsproviding D.C. power for said capacitor and switching means.